Does a ‘good’ Air Quality Index rating mean it’s safe to be outdoors?

By Aaron Schrank/LAist

Originally published Jan. 14, 2025

Angelenos still bothered by the smell of smoke may be surprised to see that Air Quality Index numbers returned to “healthy” or “moderate” levels this week in L.A. County, but they shouldn’t breathe a sigh of relief just yet. 

Public health officials and experts say the AQI, while helpful, fails to fully account for the health risks associated with destructive urban fires and high winds. 

That’s because AQI is calculated based on common air pollutants like tiny particulate matter and smog, but it doesn’t measure many other toxins currently in the air — things like asbestos, copper, or lead from structures that have burned. 

“The AQI is not reliable for this situation,” said Anthony Wexler, director of the Air Quality Research Center at the University of California-Davis. “And you can’t see it. So looking around and saying, ‘Oh, it looks, looks clear.’ That doesn’t tell you anything.”

To understand the health risks and stay safe, residents need to consider more than just the AQI metric, like the toxicity of the smoke and ash coming into their neighborhoods.

What’s the AQI?

The Air Quality Index, or AQI, is the primary system used to warn the public when air pollution is dangerous. It’s based on data collected from 5,000 air monitors across the country. The index ranges from 0 to 500 and uses a color coded system:

As of Monday evening, the AQI was between 30 and 60 throughout L.A. County — in the “good” or “moderate” range — according to data from the South Coast Air Quality Management District.

That might seem counterintuitive to many in the Los Angeles area, particularly near Pacific Palisades on the westside and Altadena in the east, where firefighters are still working to extinguish massive fires.

What AQI doesn’t measure

The AQI measures particulate matter in the air, but it doesn’t measure how toxic those particles are, Wexler said. It doesn’t measure the amount of plastic, copper, lead, or asbestos you’re breathing when you step outside. 

“The particles and gases in the air right now are much more toxic than normal because they are from burning houses which have all kinds of stuff which we don’t usually have in our air,” Wexler said. “And that stuff is bound to be much more toxic than what’s usually in the air.”

It also fails to account for most ash particles, which are typically too large to be detected by air quality instruments, but contain the toxic chemicals and may cause health problems, experts said. 

“The wind is spreading a lot of ash that is not detected in AQI,” said Nahal Mogharabi, who works with the South Coast Air Quality Management District. “These ash particles… are typically visible to the naked eye, either in the air or on outdoor surfaces.”

The toxic chemicals in the ash are the same ones that are in the smoke. 

“There’s a lot of chemicals involved — things like the plastics, toxic metals, asbestos,” said Mary Johnson, a research scientist at Harvard T.H. Chan School of Public Health. “A lot of things in our environment, when they do combust, they can be toxic.”

So even when the AQI looks “good,” Angelenos could still be exposed to materials that are harmful to their health. 

“There’s just tons of stuff out there and all the black smoky crap — that’s really bad stuff to be inhaling,” Wexler said.

Other metrics

According to the experts, one reason AQI levels had returned to normal early this week was because the fires were producing less smoke.

“Smoke production from the Eaton and Palisades fires has decreased significantly as evident from satellite imagery, webcam imagery, and local fine particle pollution monitors,” Mogharabi said. “This unfortunately may change as strong Santa Ana winds return to the area.”

L.A. County residents should take precautions whenever they smell smoke, see ash, or are in an area with high AQI levels. That means don’t exercise, keep your kids indoors, strap on your N95 mask and plug in your air filter. 

Local health officials have provided full lists of guidance on how to minimize your health risks.

Rather than relying solely on the Air Quality Index, Wexler recommends checking out PurpleAir Map, which tells you where the plumes of smoke are in real time. 

“If the plume is where you are or heading towards you, then take measures to protect yourself,” he said.

L.A. County’s Department of Public Health, which declared a local health emergencyfor L.A. County late Friday, says those risks are especially high if you have heart or lung conditions, are over 65, are pregnant or have children. 

And when it comes to using your senses to detect health risks, it’s slightly better to follow your nose than rely on what you see. 

“A better metric is your smell,” Wexler said. “Because you can smell it before you can see it. And so if it smells like smoke, it’s there or it’s coming.”

Winds will make things worse

Winds are expected to pick up Tuesday, but aren’t likely to be as strong as last week when they reached speeds of between 80 and 100 mph. The National Weather Service is warning of wind gusts between 55 and 70 mph this week.

A red flag warning is in effect throughout wide swaths of L.A. and Ventura counties Tuesday morning and will continue through Wednesday. 

Public health officials say it’s hard to predict where ash will travel or how air quality will be affected by harsh winds, but L.A. County residents should be alert. Those who are closer to the ongoing wildfires are generally at increased risk of breathing in toxins, but wind direction is also a major factor.

In short, it’s better if the wind if blowing away from you.

“Smoke and ash can harm everyone, even those who are healthy,” said Muntu Davis, health officer for L.A. County.

This report is reprinted with permission from Southern California Public Radio. © 2024 Southern California Public Radio. All rights reserved.